Hydraulic systems for operating spoilers in aircraft employ a piston type hydraulic actuator and a metering valve for alternately directing and exhausting fluid under pressure to and from each side of the actuator piston for extending and retracting the actuator piston rod to cause raising and lowering of the spoiler and also to hold the spoiler in selected positions. It is a known practice in such systems to mount a lock valve between the metering valve and the retract side of the piston to prevent raising of the spoiler by aerodynamic forces thereon in case either system pressure fails or external forces tending to raise the spoiler causes retract pressure to exceed system pressure when the latter is below the normal operating pressure.
It is also a known practice in hydraulic systems to provide a thermal relief valve between the retract side of the actuator piston and the system reservoir for relieving excess pressure on the retract side of the piston caused by thermal expansion of the hydraulic fluid when the lock valve is closed. The thermal relief valve may also function as an overload relief valve to relieve excess fluid pressure in the retract side of the piston generated by forces on the spoiler tending to extend the piston when the lock valve is closed.
In such known practice the relief valves have been arranged so as to be always subject to the full differential between reservoir pressure and the desired relief pressure. Thus, in a 3,000 PSI working pressure system the relief valve opening pressure may be set at 3,800 PSI. Because reservoir pressure is near zero there is always about 3,800 PSI differential across the relief valve when it opens. This results in high velocity flow across the relief valve seat each time the relief valve opens which in turn causes rapid erosion of the seat resulting in constant leakage when the relief valve is closed.
During flight of the aircraft while hydraulic system pressure is utilized for holding the spoiler down, aerodynamic forces on the spoiler will tend to raise it. Frequently this imposes enough extending force on the actuator piston to raise the pressure on the retract side close to, or even higher than, system pressure when the latter has been temporarily reduced due to flow demand of other components of the hydraulic system. When this occurs the lock valve closes to trap the fluid in the retract chamber and prevent extension of the piston and raising of the spoiler. However if, due to surges in the system pressure, the pressure in the retract side pulses to a pressure that exceeds the opening pressure of the relief valve, the latter will open to prevent a further rise in pressure in the retract chamber. The pressure in the retract chamber may pulse to the relief pressure many times during a flight and this may result in rapid erosion of the seat and leakage across the relief valve as aforesaid. If for any reason the system pressure should then fail during flight there will be loss of fluid across the relief valve from the retract side of the actuator piston and the spoiler can then raise and/or flutter with possible disastrous consequences to the aircraft.